1. Field of the Invention
The present invention relates to a reciprocating internal combustion engine, and particularly to a reciprocating internal combustion engine having a geometric compression ratio of 13.0 or greater.
2. Description of the Background Art
In a reciprocating internal combustion engine, a technique intended to improve flame propagation characteristics based on a configuration of a combustion chamber has been proposed. For example, JP 2007-154827A proposes a technique of forming a spherical-shaped cavity on a top surface of a piston so as to uniformize flame propagation to improve combustion efficiency.
Further, as means for improving fuel economy, a technique of setting a geometric compression ratio at a higher value to improve thermal efficiency has been proposed. However, if the geometric compression ratio is simply set at a higher value, there arise problems that no improvement in fuel economy can be obtained in some cases, and knocking is more likely to occur.
It is considered that one of the causes of the problems roots in flame propagation characteristics. Specifically, given that an engine displacement is constant, as the geometric compression ratio is set at a higher value, a volume of a combustion chamber becomes smaller, and thereby a flame comes into interference with an inner wall defining the combustion chamber, such as a top surface of a piston, at an earlier timing to cause deterioration in thermal efficiency due to cooling loss resulting from the interference, and occurrence of abnormal combustion. In order to improve flame propagation characteristics, it is contemplated to provide a cavity on a top surface of a piston, as disclosed in the above patent publication. However, if such a cavity is simply provided, the geometric compression ratio will become smaller, resulting in insufficient improvement in fuel economy.